With the demise of the steam locomotive and the advent of the diesel electric locomotive, the railroads found that dissolved minerals in the engine cooling water, such as calcium and silicates, caused corrosion and scale formation within the cooling system. For example radiators became clogged, cylinder liners were corroded, and the scale and corrosion deposits reduced the heat exchange efficiency of the cooling system. To avoid these problems, water treatment chemicals were formulated to reduce the corrosive effects of the dissolved minerals and ameliorate scale formation. The water treatment formulations that have proved most effective and have achieved wide acceptance are alkaline solutions containing borate and nitrite anions. Formulations of this type are fully disclosed, for example in U.S. Pat. Nos. 2,815,328, 2,972,581, and 3,948,792 the teachings of which are incorporated herein by reference.
While the prior art formulations are acceptable in their chemical performance, there is no convenient or reliable method by which the concentration of the treatment chemicals in the coolant water can be determined. Since make-up water must be added from time to time to compensate for inevitable coolant losses, the level of treatment chemicals may fall below desired levels or, in the alternative, treatment chemicals may be wasted by adding them in excessive amounts. Sometimes this problem has been addressed by adding a pH indicator, particularly phenolphthalein, to the treatment chemicals. Since the corrosion inhibitors are most effective in a pH range of from about 8.5 to 11 and they are formulated to adjust the coolant to that level, phenolphthalein will cause the coolant to change from red to colorless when the pH falls below 8 which can be observed, for example, through a sight glass. The use of phenolphthalein does not really solve the problem, however, since the pH must fall below the preferred level before any indication is given, since no indication is given if the treatment level is to high, and since the color intensity of the phenolphthalein diminishes with use in the environment.